Bound states in ultrastrong waveguide QED

Abstract

We discuss the properties of bound states in finite-bandwidth waveguide QED beyond the rotating wave approximation or excitation-number-conserving light-matter coupling models. Therefore, we extend the standard calculations to a broader range of light-matter strengths, in particular, in the so-called ultrastrong coupling regime. We do this using the polaron technique. Our main results are as follows: We compute the spontaneous emission rate, which is renormalized as compared with the Fermi golden rule formula. We generalize the existence criteria for bound states, their properties, and their role in the qubit thermalization. We discuss effective spin-spin interactions through both vacuum fluctuations and bound states. Finally, we sketch a perfect state-transfer protocol among distant emitters mediated by bound states.

Distant emitters in ultrastrong waveguide QED: Ground-state properties and non-Markovian dynamics

“…In general, the SBM can not be diagonalized, but a useful approach based on the introduction of a variational displaced 053701-2 oscillator basis can be employed for the study of static and dynamical properties of such systems [19,31,47,[61][62][63]. The accuracy of the polaron approach has been tested in several works for one-qubit [27,[64][65][66] and multiqubit systems [67,68].…”

“…Not only the ground state but the first excited states are well captured by the polaron Ansatz [31,62,63,72]. It is convenient to work in the polaron picture (1):…”

A proposal for using molecular spin qudits as quantum simulators of light–matter interactions

“…Hamiltonian (eqn (2)) describes the interaction between a radiation field and a twolevel system, such as an atom or a spin 1/2 particle. Important applications come from the context of quantum computing, where the Rabi Hamiltonian is used to model the coupling of quantized photons in superconducting wave-guide resonators 69,70 with superconducting qubits or spin systems. 14 Behind its apparent simplicity, our target model can reveal interesting physics in different ranges of parameters.…”

Distant emitters in ultrastrong waveguide QED: Ground-state properties and non-Markovian dynamics

“…Not only the ground state but the first excited states are well captured by the polaron ansatz [29,51,57,58]. It is convenient to work in the polaron picture (1): H P = U † P HU P ≈ H eff , which can be approximated by [51]…”